What is Catholic Mass?
Found in the Christian religion, within the branch of Roman Catholicism, mass is the collection of prayers and ceremonies that make up the service of the Eucharist, which is a memorial that Jesus Christ instituted at the Last Supper to commemorate his death on the cross for the sins of people.
The name comes from the first period, while Greek was still the Christian language at Rome. The common name for the ceremony was "Eucharistia," used both for the consecrated bread and wine and for the whole service. The word carries the meaning of "giving thanks."
The "eucharist" remains the normal name for the sacrament throughout Catholic theology, but is gradually superseded by Missa for the whole rite.
First use of "mass" in the Catholic Church
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The first certain use of "mass" is by St. Ambrose (d. 397). He writes to his sister Marcellina describing the troubles of the Arians in the years 385 and 386, when the soldiers were sent to break up the service in his church:
"The next day (it was a Sunday) after the lessons and the tract, having dismissed the catechumens, I explained the creed [symbolum tradebam] to some of the competents [people about to be baptized] in the baptistry of the basilica. There I was told suddenly that they had sent soldiers to the Portiana basilica. . . . But I remained at my place and began to say Mass [missam facere coepi]. While I offer [dum ofero], I hear that a certain Castulus has been seized by the people" (Ep., I, xx, 4-5).
It will be noticed that missa here means the Eucharistic Service proper, the Liturgy of the Faithful only, and does not include that of the Catechumens. Ambrose uses the word as one in common use and well known. There is another, still earlier, but very doubtfully authentic instance of the word in a letter of Pope Pius I (from c. 142 to c. 157):
"Euprepia has handed over possession of her house to the poor, where . . . we make Masses with our poor" (cum pauperibus nostris . . . missas agimus" — Pii I, Ep. I, in Galland, "Bibl. vet. patrum", Venice, 1765, I, 672). The authenticity of the letter, however, is very doubtful. If Missa really occurred in the second century in the sense it now has, it would be surprising that it never occurs in the third. We may consider St. Ambrose as the earliest certain authority for it.
From the fourth century the term becomes more and more common. For a time it occurs nearly always in the sense of dismissal. St. Augustine (d. 430) says: "After the sermon the dismissal of the catechumens takes place" . The Synod of Lerida in Spain (524) declares that people guilty of incest may be admitted to church "usque ad missam catechumenorum", that is, till the catechumens are dismissed.
The same expression occurs in the Synod of Valencia at about the same time , in Hincmar of Reims (d. 882), etc. Etheria (fourth century) calls the whole service, or the Liturgy of the Faithful, missa constantly . So also Innocent I (401-17) in Ep., xvii, 5, P.L., XX, 535, Leo I (440-61), in Ep., ix, 2, P.L., LIV, 627. Although from the beginning the word Missa usually means the Eucharistic Service or some part of it, we find it used occasionally for other ecclesiastical offices too. In St. Benedict's (d. 543) Rule fiant missae is used for the dismissal at the end of the canonical hours (chap., xvii, passim).
In the Leonine Sacramentary (sixth cent.), the word in its present sense is supposed throughout. The title, "Item alia", at the head of each Mass means "Item alia missa". The Gelasian book (sixth or seventh cent. Cf. ibid.) supplies the word: "Item alia missa", "Missa Chrismatis", "Orationes ad missa [sic] in natale Sanctorum", and so on throughout. From that time it becomes the regular, practically exclusive, name for the Holy Liturgy in the Roman and Gallican Rites.
The meaning of "mass" in the Catholic Church
The origin and first meaning of the word, once much discussed, is not really doubtful. We may dismiss at once such fanciful explanations as that missa is the Hebrew missah ("oblation" — so Reuchlin and Luther), or the Greek myesis ("initiation"), or the German Mess ("assembly", "market"). Nor is it the participle feminine of mittere, with a noun understood ("oblatio missa ad Deum", "congregatio missa", i.e., dimissa — so Diez, "Etymol. Wörterbuch der roman. Sprachen", 212, and others). It is a substantive of a late form for missio.
There are many parallels in medieval Latin, collecta, ingressa, confessa, accessa, ascensa — all for forms in -io. It does not mean an offering (mittere, in the sense of handing over to God), but the dismissal of the people, as in the versicle: "Ite missa est" (Go, the dismissal is made). It may seem strange that this unessential detail should have given its name to the whole service. But there are many similar cases in liturgical language. Communion, confession, breviary are none of them names that express the essential character of what they denote. In the case of the word missa we can trace the development of its meaning step by step.
We have seen it used by St. Augustine, synods of the sixth century, and Hincmar of Reims for "dismissal". Missa Catechumenorum means the dismissal of the catechumens. It appears that missa fit or missa est was the regular formula for sending people away at the end of a trial or legal process. Avitus of Vienne (d. 523) says: "In churches and palaces or law-courts the dismissal is proclaimed to be made [missa pronuntiatur], when the people are dismissed from their attendance" (Ep. i).
So also St. Isidore of Seville: "At the time of the sacrifice the dismissal is [missa tempore sacrificii est] when the catechumens are sent out, as the deacon cries: If any one of the catechumens remain, let him go out: and thence it is the dismissal [et inde missa]" ("Etymol.", VI, xix, in P.L., LXXXII, 252). As there was a dismissal of the catechumens at the end of the first part of the service, so was there a dismissal of the faithful (the baptized) after the Communion.
There were, then, a missa catechumenorum and a missa fidelium, both, at first, in the sense of dismissals only. So Florus Diaconus (d. 860):
"Missa is understood as nothing but dimissio, that is, absolutio, which the deacon pronounces when the people are dismissed from the solemn service. The deacon cried out and the catechumens were sent [mittebantur], that is, were dismissed outside [id est, dimittebantur foras]. So the missa caechumenorum was made before the action of the Sacrament (i. e., before the Canon Actionis), the missa fidelium is made "
— note the difference of tense; in Florus's time the dismissal of the catechumens had ceased to be practised —" after the consecration and communion" [post confectionem et participationem] (P.L., CXIX 72).
How the word gradually changed its meaning from dismissal to the whole service, up to and including the dismissal, is not difficult to understand. In the texts quoted we see already the foundation of such a change.
To stay till the missa catechumenorum is easily modified into: to stay for, or during, the missa catechumenorum. So we find these two missae used for the two halves of the Liturgy. Ivo of Chartres (d. 1116) has forgotten the original meaning, and writes: "Those who heard the missa catechumenorum evaded the missa sacramentorum" (Ep. ccxix, in P.L., CLXII, 224).
The two parts are then called by these two names; as the discipline of the catechumenate is gradually forgotten, and there remains only one connected service, it is called by the long familiar name missa, without further qualification.
We find, however, through the Middle Ages the plural miss, missarum solemnia, as well as missae sacramentum and such modified expressions also. Occasionally the word is transferred to the feast-day. The feast of St. Martin, for instance, is called Missa S. Martini. It is from this use that the German Mess, Messtag, and so on are derived. The day and place of a local feast was the occasion of a market.
It should be noted that the name Mass (missa) applies to the Eucharistic service in the Latin rites only. Neither in Latin nor in Greek has it ever been applied to any Eastern rite. For them the corresponding word is Liturgy (liturgia). It is a mistake that leads to confusion, and a scientific inexactitude, to speak of any Eastern Liturgy as a Mass.
The Origin of the Mass in the Catholic Church
The Western Mass, like all Liturgies, begins, of course, with the Last Supper. What Christ then did, repeated as he commanded in memory of Him, is the nucleus of the Mass. As soon as the Faith was brought to the West the Holy Eucharist was celebrated here, as in the East. At first the language used was Greek. Out of that earliest Liturgy, the language being changed to Latin, developed the two great parent rites of the West, the Roman and the Gallican.
Of these two the Gallican Mass may be traced without difficulty. It is so plainly Antiochene in its structure, in the very text of many of ifs prayers, that we are safe in accounting for it as a translated form of the Liturgy of Jerusalem-Antioch, brought to the West at about the time when the more or less fluid universal Liturgy of the first three centuries gave place to different fixed rites.
Mass in the writings of the early church fathers
The origin of the Roman Mass, on the other hand, is a most difficult question, We have here two fixed and certain data: the Liturgy in Greek described by St. Justin Martyr (d. c. 165), which is that of the Church of Rome in the second century, and, at the other end of the development, the Liturgy of the first Roman Sacramentaries in Latin, in about the sixth century. The two are very different. Justin's account represents a rite of what we should now call an Eastern type, corresponding with remarkable exactness to that of the Apostolic Constitutions.
The Leonine and Gelasian Sacramentaries show us what is practically our present Roman Mass. How did the service change from the one to the other? It is one of the chief difficulties in the history of liturgy. During the last few years, especially, all manner of solutions and combinations have been proposed. We will first note some points that are certain, that may serve as landmarks in an investigation.
Justin Martyr, Clement of Rome, Hippolytus (d. 235), and Novatian (c. 250) all agree in the Liturgies they describe, though the evidence of the last two is scanty (Probst, "Liturgie der drei ersten christl. Jahrhdte"; Drews, "Untersuchungen über die sogen. clement. Liturgie"). Justin gives us the fullest Liturgical description of any Father of the first three centuries (Apol. I, lxv, lxvi).
He describes how the Holy Eucharist was celebrated at Rome in the middle of the second century; his account is the necessary point of departure, one end of a chain whose intermediate links are hidden. We have hardly any knowledge at all of what developments the Roman Rite went through during the third and fourth centuries. This is the mysterious time where conjecture may, and does, run riot.
By the fifth century we come back to comparatively firm ground, after a radical change. At this time we have the fragment in Pseudo-Ambrose, "De sacramentis" (about 400. Cf. P.L., XVI, 443), and the letter of Pope Innocent I (401-17) to Decentius of Eugubium (P.L., XX, 553). In these documents we see that the Roman Liturgy is said in Latin and has already become in essence the rite we still use. A few indications of the end of the fourth century agree with this. A little later we come to the earliest Sacramentaries (Leonine, fifth or sixth century; Gelasian, sixth or seventh century) and from then the history of the Roman Mass is fairly clear.
The fifth and sixth centuries therefore show us the other end of the chain. For the interval between the second and fifth centuries, during which the great change took place, although we know so little about Rome itself, we have valuable data from Africa.
There is every reason to believe that in liturgical matters the Church of Africa followed Rome closely. We can supply much of what we wish to know about Rome from the African Fathers of the third century, Tertullian (d. c. 220), St. Cyprian (d. 258), the Acts of St. Perpetua and St. Felicitas (203), St. Augustine (d. 430) (see Cabrol, "Dictionnaire d' archéologie", I, 591-657).
The question of the change of language from Greek to Latin is less important than if might seem. It came about naturally when Greek ceased to be the usual language of the Roman Christians. Pope Victor I (190-202), an African, seems to have been the first to use Latin at Rome, Novatian writes Latin.
By the second half of the third century the usual liturgical language at Rome seems to have been Latin (Kattenbusch, "Symbolik", II, 331), though fragments of Greek remained for many centuries. Other writers think that Latin was not finally adopted till the end of the fourth century (Probst, "Die abendländ. Messe", 5; Rietschel, "Lehrbuch der Liturgik", I, 337).
No doubt, for a time both languages were used. The question is discussed at length in C. P. Caspari, "Quellen zur Gesch. des Taufsymbols u. der Glaubensregel" (Christiania, 1879), III, 267 sq. The Creed was sometimes said in Greek, some psalms were sung in that language, the lessons on Holy Saturday were read in Greek and Latin as late as the eighth century (Ordo Rom., I, P.L., LXXVIII, 966-68, 955).
There are still such fragments of Greek ("Kyrie eleison", "Agios O Theos") in the Roman Mass. But a change of language does not involve a change of rite. Novatian's Latin allusions to the Eucharistic prayer agree very well with those of Clement of Rome in Greek, and with the Greek forms in Apost. Const., VIII (Drews, op. cit., 107-22). The Africans, Tertullian, St. Cyprian, etc., who write Latin, describe a rite very closely related to that of Justin and the Apostolic Constitutions (Probst, op. cit., 183-206; 215-30).
The Gallican Rite, as in Germanus of Paris (Duchesne, "Origines du Culte", 180-217), shows how Eastern — how "Greek" — a Latin Liturgy can be. We must then conceive the change of language in the third century as a detail that did not much affect the development of the rite. No doubt the use of Latin was a factor in the Roman tendency to shorten the prayers, leave out whatever seemed redundant in formulas, and abridge the whole service. Latin is naturally terse, compared with the rhetorical abundance of Greek. This difference is one of the most obvious distinctions between the Roman and the Eastern Rites.
If we may suppose that during the first three centuries there was a common Liturgy throughout Christendom, variable, no doubt, in details, but uniform in all its main points, which common Liturgy is represented by that of the eighth book of the Apostolic Constitutions, we have in that the origin of the Roman Mass as of all other liturgies. There are, indeed, special reasons for supposing that this type of liturgy was used at Rome.
The chief authorities for it (Clement, Justin, Hippolytus, Novatian) are all Roman. Moreover, even the present Roman Rite, in spite of later modifications, retains certain elements that resemble those of the Apost. Const. Liturgy remarkably. For instance, at Rome there neither is nor has been a public Offertory prayer. The "Oremus" said just before the Offertory is the fragment of quite another thing, the old prayers of the faithful, of which we still have a specimen in the series of collects on Good Friday.
The Present Roman Mass
It is not the object of this paragraph to give instruction as to how the Roman Mass is celebrated. The very complicated rules of all kinds, the minute rubrics that must be obeyed by the celebrant and his ministers, all the details of coincidence and commemoration — these things, studied at length by students before they are ordained, must be sought in a book of ceremonial (Le Vavasseur, quoted in the bibliography, is perhaps now the best).
It will be sufficient here to give a general outline of the arrangement. The ritual of the Mass is affected by (1) the person who celebrates, (2) the day or the special occasion on which it is said, (3) the kind of Mass (high or low) celebrated. But in all cases the general scheme is the same. The normal ideal may be taken as high Mass sung by a priest on an ordinary Sunday or feast that has no exceptional feature.
Normally, Mass must be celebrated in a consecrated or blessed Church (private oratories or even rooms are allowed for special reasons: see Le Vavasseur, I, 200-4) and at a consecrated altar (or at least on a consecrated altar-stone), and may be celebrated on any day in the year except Good Friday (restrictions are made against private celebrations on Holy Saturday and in the case of private oratories for certain great feasts) at any time between dawn and midday.
A priest may say only one Mass each day, except that on Christmas Day he may say three, and the first may (or rather, should) then be said immediately after midnight. In some countries (Spain and Portugal) a priest may also celebrate three times on All Souls' Day (2 November). Bishops may give leave to a priest to celebrate twice on Sundays and feasts of obligation, if otherwise the people could not fulfil their duty of hearing Mass.
In cathedral and collegiate churches, as well as in those of religious orders who are bound to say the Canonical Hours every day publicly, there is a daily Mass corresponding to the Office and forming with it the complete cycle of the public worship of God. This official public Mass is called the conventual Mass; if possible it should be a high Mass, but, even if it be not, it always has some of the features of high Mass.
The time for this conventual Mass on feasts and Sundays is after Terce has been said in choir. On Simples and feriæ the time is after Sext; on feriæ of Advent, Lent, on Vigils and Ember days after None. Votive Masses and the Requiem on All Souls' Day are said also after None; but ordinary requiems are said after Prime. The celebrant of Mass must be in the state of grace, fasting from midnight, free of irregularity and censure, and must observe all the rubrics and laws concerning the matter (azyme bread and pure wine), vestments, vessels, and ceremony.
The scheme of high Mass is this: the procession comes to the altar, consisting of thurifer, acolytes, master of ceremonies, subdeacon, deacon, and celebrant, all vested as the rubrics direct. First, the preparatory prayers are said at the foot of the altar; the altar is incensed, the celebrant reads at the south (Epistle) side the Introit and Kyrie. Meanwhile the choir sing the Introit and Kyrie. On days on which the "Te Deum" is said in the office, the celebrant intones the "Gloria in excelsis", which is continued by the choir.
Meanwhile he, the deacon, and subdeacon recite it, after which they may sit down till the choir has finished. After the greeting "Dominus vobiscum", and its answer "Et cum spiritu tuo", the celebrant chants the collect of the day, and after it as many more collects as are required either to commemorate other feasts or occasions, or are to be said by order of the bishop, or (on lesser days) are chosen by himself at his discretion from the collection in the Missal, according to the rubrics. The subdeacon chants the Epistle and the choir sings the Gradual.
Both are read by the celebrant at the altar, according to the present law that he is also to recite whatever is sung by any one else. He blesses the incense, says the "Munda Cor meum" prayer, and reads the Gospel at the north (Gospel) side. Meanwhile the deacon prepares to sing the Gospel. He goes in procession with the subdeacon, thurifer, and acolytes to a place on the north of the choir, and there chants it, the subdeacon holding the book, unless an ambo be used. If there is a sermon, if should be preached immediately after the Gospel.
This is the traditional place for the homily, after the lessons (Justin Martyr, "I Apolog.", lxvii, 4). On Sundays and certain feasts the Creed is sung next, just as was the Gloria. At this point, before or after the Creed (which is a later introduction, as we have seen), ends in theory the Mass of the Catechumens. The celebrant at the middle of the altar chants "Dominus vobiscum" and "Oremus" — the last remnant of the old prayers of the faithful.
Then follows the Offertory. The bread is offered to God with the prayer "Suscipe sancte Pater"; the deacon pours wine into the chalice and the subdeacon water. The chalice is offered by the celebrant in the same way as the bread (Offerimus tibi Domine), after which the gifts, the altar, the celebrant, ministers, and people are all incensed. Meanwhile the choir sings the Offertory. The celebrant washes his hands saying the "Lavabo".
After another offertory prayer (Suscipe sancta Trinitas), and an address to the people (Orate fratres) with its answer, which is not sung (it is a late addition), the celebrant says the secrets, corresponding to the collects. The last secret ends with an Ekphonesis (Per omnia sæcula sæculorum). This is only a warning of what is coming. When prayers began to be said silently, it still remained necessary to mark their ending, that people might know what is going on. So the last clauses were said or sung aloud. This so-called Ekphonesis is much developed in the Eastern rites.
In the Roman Mass there are three cases of it — always the words: "Per omnia sæcula sæculorum", to which the choir answers "Amen". After the Ekphonesis of the Secret comes the dialogue, "Sursum Cords", etc., used with slight variations in all rites, and so the beginning of the Eucharistic prayer which we call the Preface, no longer counted as part of the Canon. The choir sings and the celebrant says the Sanctus. Then follows the Canon, beginning "Te Igitur" and ending with an ekphonesis before the Lord's Prayer.
The Lord's Prayer follows, introduced by a little clause (Præceptis salutaribus moniti) and followed by an embolism, said silently and ending with the third ekphonesis. The Fraction follows with the versicle "Pax domini sit semper vobiscum", meant to introduce the kiss of peace. The choir sings the Agnus Dei, which is said by the celebrant together with the first Communion prayer, before he gives the kiss to the deacon.
He then says the two other Communion prayers, and receives Communion under both kinds. The Communion of the people (now rare at high Mass) follows. Meanwhile the choir sings the Communion. The chalice is purified and the post-Communions are sung, corresponding to the collects and secrets. Like the collects, they are introduced by the greeting "Dominus vobiscum" and its answer, and said at the south side. After another greeting by the celebrant the deacon sings the dismissal.
There still follow, however, three later additions, a blessing by the celebrant, a short prayer that God may be pleased with the sacrifice (Placeat tibi) and the Last Gospel, normally the beginning of St. John. The procession goes back to the sacristy.
This high Mass is the norm; it is only in the complete rite with deacon and subdeacon that the ceremonies can be understood. Thus, the rubrics of the Ordinary of the Mass always suppose that the Mass is high.
Low Mass, said by a priest alone with one server, is a shortened and simplified form of the same thing. Its ritual can be explained only by a reference to high Mass. For instance, the celebrant goes over to the north side of the altar to read the Gospel, because that is the side to which the deacon goes in procession at high Mass; he turns round always by the right, because at high Mass he should not turn his back to the deacon and so on.
A sung Mass (missa Cantata) is a modern compromise. It is really a low Mass, since the essence of high Mass is not the music but the deacon and subdeacon. Only in churches which have no ordained person except one priest, and in which high Mass is thus impossible, is it allowed to celebrate the Mass (on Sundays and feasts) with most of the adornment borrowed from high Mass, with singing and (generally) with incense.
The Sacred Congregation of Rites has on several occasions (9 June, 1884; 7 December, 1888) forbidden the use of incense at a Missa Cantata; nevertheless, exceptions have been made for several dioceses, and the custom of using it is generally tolerated (Le Vavasseur, op. cit., I, 514-5). In this case, too, the celebrant takes the part of deacon and subdeacon; there is no kiss of peace.
The ritual of the Mass is further affected by the dignity of the celebrant, whether bishop or only priest. There is something to be said for taking the pontifical Mass as the standard, and explaining that of the simple priest as a modified form, just as low Mass is a modified form of high Mass. On the other hand historically the case is not parallel throughout; some of the more elaborate pontifical ceremony is an after-thought, an adornment added later.
Here it need only be said that the main difference of the pontifical Mass (apart from some special vestments) is that the bishop remains at his throne (except for the preparatory prayers at the altar steps and the incensing of the altar) till the Offertory; so in this case the change from the Mass of the Catechumens to that of the Faithful is still clearly marked. He also does not put on the maniple till after the preparatory prayers, again an archaic touch that marks them as being outside the original service.
At low Mass the bishop's rank is marked only by a few unimportant details and by the later assumption of the maniple. Certain prelates, not bishops, use some pontifical ceremonies at Mass. The pope again has certain special ceremonies in his Mass, of which some represent remnants of older customs, Of these we note especially that he makes his Communion seated on the throne and drinks the consecrated wine through a little tube called fistula.
Durandus (Rationale, IV, i) and all the symbolic authors distinguish various parts of the Mass according to mystic principles. Thus it has four parts corresponding to the four kinds of prayer named in I Tim., ii, 1. It is an Obsecratio from the Introit to the Offertory, an Oratio from the Offertory to the Pater Noster, a Postulatio to the Communion, a Gratiarum actio from then to the end (Durandus, ibid).
The Canon especially has been divided according to all manner of systems, some very ingenious. But the distinctions that are really important to the student of liturgy are, first the historic division between the Mass of the Catechumens and Mass of the Faithful, already explained, and then the great practical distinction between the changeable and unchangeable parts. The Mass consists of an unchanged framework into which at certain fixed points the variable prayers, lessons, and chants are fitted.
The two elements are the Common and the Proper of the day (which, however, may again be taken from a common Mass provided for a number of similar occasions, as are the Commons of various classes of saints). The Common is the Ordinary of the Mass (Ordinarium Missae), now printed and inserted in the Missal between Holy Saturday and Easter Day. Every Mass is fitted into that scheme; to follow Mass one must first find that. In it occur rubrics directing that something is to be said or sung, which is not printed at this place.
The first rubric of this kind occurs after the incensing at the beginning: "Then the Celebrant signing himself with the sign of the Cross begins the Introit." But no Introit follows. He must know what Mass he is to say and find the Introit, and all the other proper parts, under their heading among the large collection of masses that fill the book.
These proper or variable parts are first the four chants of the choir, the Introit, Gradual (or tract, Alleluia, and perhaps after it a Sequence), Offertory, and Communion; then the lessons (Epistle, Gospel, sometimes Old Testament lessons too), then the prayers said by the celebrant (Collect, Secret, post-communion; often several of each to commemorate other feasts or days).
By fitting these into their places in the Ordinary the whole Mass is put together. There are, however, two other elements that occupy an intermediate place between the Ordinary and the Proper. These are the Preface and a part of the Canon. We have now only eleven prefaces, ten special ones and a common preface. They do not then change sufficiently to be printed over and over again among the proper Masses, so all are inserted in the Ordinary; from them naturally the right one must be chosen according to the rubrics.
In the same way, five great feasts have a special clause in the Communicantes prayer in the Canon, two (Easter and Whitsunday) have a special "Hanc Igitur" prayer, one day (Maundy Thursday) affects the "Qui pridie" form. These exceptions are printed after the corresponding prefaces; but Maundy Thursday, as it occurs only once, is to be found in the Proper of the day.
It is these parts of the Mass that vary, and, because of them, we speak of the Mass of such a day or of such a feast. To be able to find the Mass for any given day requires knowledge of a complicated set of rules. These rules are given in the rubrics at the beginning of the Missal. In outline the system is this. First a Mass is provided for every day in the year, according to the seasons of the Church. Ordinary week days (feriæ) have the Mass of the preceding Sunday with certain regular changes; but feriæ of Lent, rogation and ember days, and vigils have special Masses. All this makes up the first part of the Missal called Proprium de tempore.
The year is then overladen, as it were, by a great quantity of feasts of saints or of special events determined by the day of the month (these make up the Proprium Sanctorum). Nearly every day in the year is now a feast of some kind; often there are several on one day. There is then constantly coincidence (concurrentia) of several possible Masses on one day. There are cases in which two or more conventual Masses are said, one for each of the coinciding offices. Thus, on feriæ that have a special office, if a feast occurs as well, the Mass of the feast is said after Terce, that of the feria after None. If a feast falls on the Eve of Ascension Day there are three Conventual Masses — of the feast after Terce, of the Vigil after Sext, of Rogation day after None.
But, in churches that have no official conventual Mass and in the case of the priest who says Mass for his own devotion, one only of the coinciding Masses is said, the others being (usually) commemorated by saying their collects, secrets, and post-Communions after those of the Mass chosen. To know which Mass to choose one must know their various degrees of dignity. All days or feasts are arranged in this scale: feria, simple, semidouble double, greater double, double of the second class, double of the first class.
The greater feast then is the one kept: by transferring feasts to the next free day, it is arranged that two feasts of the same rank do not coincide. Certain important days are privileged, so that a higher feast cannot displace them. Thus nothing can displace the first Sundays of Advent and Lent, Passion and Palm Sundays. These are the so-called first-class Sundays. In the same way nothing can displace Ash Wednesday or any day of Holy Week.
Other days (for instance the so-called second-class Sundays, that is the others in Advent and Lent, and Septuagesima, Sexagesima, and Quinquagesima) can only be replaced by doubles of the first class. Ordinary Sundays count as semidoubles, but have precedence over other semidoubles. The days of an octave are semidoubles; the octave day is a double. The octaves of Epiphany, Easter, and Pentecost (the original three greatest feasts of all) are closed against any other feast.
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The Catholic Encyclopedia (1914 ed. in the public domain), "Mass Liturgy" with minor edits.